Ethers are commercially important compounds, and a wide variety are known. Many are used with respect to solvents, propellants, fillers, food additives, fuel additives, cleaners, health care formulations, manufacture of polymers, etc. Ethers include polyether oligomers and polymers such as the polyoxyalkylene oligomers and polymers. Polyethyleneglycols, based upon repeating units of oxyethylene, are a type of polyoxyalkylene oligomer and are very widely known and used. These incorporate multiple ether linkages along the body of the molecule. These may also have one or more terminal ether groups. Polyoxyalkylene products often are commercially available as a mixture containing a distribution of oligomers and/or polymers with varying number of repeating units and a corresponding variation in molecular weight.
Linear polyoxyalkylene materials with terminal ether groups at each end are commonly used as solvents in the chemical industry. These solvents are also referred to as polyalkylene glycol dialkyl ethers and are well known as solvents for acidic gases such as carbon dioxide, hydrogen sulfide, carbonyl sulfide, carbon disulfide, mercaptans, and the like. These solvents are used to scrub such acid gases from process streams. These solvents are described in U.S. Pat. Nos. 2,649,166; 3,362,133; 3,591,641; 3,737,392; 3,824,766; 3,837,143; 4,044,100; 4,581,154; 4,741,745; 4,946,620; 4,995,888; 5,273,679; and 6,592,779. See also EP 146886. It is well known from these patent documents that the dialkyl ethers of polyoxyalkylene glycols, and in particular the dimethyl ethers, comprise mixtures of polyoxyalkylene adducts typically ranging from 1 to 9 units of the oxyethylene moiety.
The Williamson ether synthesis was developed by Alexander Williamson in 1850. This synthesis involves converting an alkoxide ion to an ether by reaction with a hydrocarbyl halide. The Williamson synthesis continues to be widely used, and indeed, is applied to the synthesis of polyoxyalkylene materials with terminal ether groups. In the manufacture of such materials, a linear polyoxyalkylene having an ether group at one end and an alcohol at the other end serves as an alkoxide precursor. In the presence of Na or a strong base, the alcohol is converted to the corresponding alkoxide. This alkoxide reacts with a hydrocarbyl halide to form the desired diether.
The synthesis of polyalkyleneglycol dimethyl ethers has been described in EP 146886 and U.S. Pat. No. 3,591,641. These patents use Na metal as a reactant to form alkoxide anion and then filter or centrifuge to remove the sodium chloride byproduct. The use of sodium metal requires extreme care in its use and necessitates venting of hydrogen gas from the reactor. Relying upon filtering or centrifuging to remove sodium chloride also results in a substantial loss of product. The captured sodium chloride also has to be cleaned before it can be discarded or recycled.
An alternative process for making polyalkyleneglycol dimethyl ethers has used NaOH as the reactant to form alkoxide. However, unlike the present invention, this early process only used a modest stoichiometric molar excess of the base relative to the alkoxide on the order of about 0.1 to 0.2 equivalent stoichiometric excess. This process suffers from yield issues. The dimethyl ether products may also suffer from color issues causing them to fail to meet color specifications consistently. For instance, color specifications might specify that the dimethyl ether product has a Gardner number of less than 3, which might serve as a qualitative measure of product purity, corresponding to a relatively clear, relatively colorless product. Yet, some product according to this process might have a Gardner number on the order of about 10, which suggests a substantial amount of impurities in the product that could affect the ability of the material in its intended end use application.
Improved methods for making ethers, particularly diethers, more particularly diethers of polyoxyalkylene materials, are highly desired.